1. Field of the Invention
The present invention relates to the technology to control polishing processes of a glass substrate with high accuracy.
2. Description of the Related Art
In the prior art, such approaches are studied that, upon performing the polishing process of the glass substrate, the polishing process should be performed with high accuracy by grasping precisely respective states of a polishing apparatus and a polished subject during the polishing process and then controlling the polishing process of the glass substrate based on detected information.
Out of them, the method of polishing the glass substrate while controlling an amount of polishing with high accuracy has an influence largely upon the plate thickness as the important quality characteristic that is required of the glass substrate products. For this reason, various methods are discussed now.
For example, in the glass substrate for the magnetic recording medium, with the progress of higher recording density in recent years, the demand for process accuracy of the glass substrate used in the magnetic recording medium is escalating. According to this demand, the glass substrate whose dimensional variation of plate thickness is small is demanded.
As the conventional methods of controlling an amount of polishing of the glass substrate, there are known
(1) the method of controlling an amount of polishing, by calculating a polishing rate of the main polishing every lot by using the polishing rate obtained when the polishing process was conducted in advance, then deciding an amount of necessary polishing by measuring a plate thickness of the glass substrate prior to the polishing by means of the weighing method or the micrometer method, then calculating a polishing time based on the polishing rate of the main polishing and the necessary polishing, and then deciding the polishing time every lot,
(2) the method of controlling an amount of polishing of the magnetic disc substrate by measuring a vertical movement of the supporting pins of the upper platen, which supports the upper platen of the polishing apparatus, by using an eddy-current sensor, or the method of controlling a plate thickness of the magnetic disc substrate by specifying a relative vertical position of the supporting pins of the upper platen with respect to the slider by using the contact type displacement sensor (see JP-A-2004-345018, for example), and
(3) the method of managing an amount of polishing, by sampling the glass substrate for the amount-of-polishing measurement during the polishing process, and then measuring an amount of reduction of mass from the glass substrate before the polishing (see JP-A-2008-825, for example).
However, in the conventional method (1), the polishing rate of the main polishing is calculated by using an empirical formula of the polishing rate. Therefore, in the situation that the polishing rate of the main polishing is out of the empirical prediction, such problems arise that the glass substrate whose thickness is excessively reduced due to the excessive polishing should be discarded, the glass substrate that could not be polished within a desired dimensional range as a target dimension due to the insufficient polishing should be polished once again, and the like. As a result, a variation in dimension of a thickness of the glass substrate is increased.
Also, in the conventional method (2), such problems arise that a labor and a cost are needed to fit a thickness measuring mechanism to the polishing apparatus, and a thickness cannot be controlled sufficiently with high accuracy due to a vibration of the apparatus, an influence of a temperature rise of the apparatus members caused by the machining, and contamination of a sensing portion of the sensor.
Also, in the conventional method (3), the polishing apparatus is stopped once in the course of the polishing process, then the glass substrate for the amount-of-polishing measurement is sampled and cleaned/dried, and then a mass must be measured. As a result, a variation in thickness dimension of the glass substrate is increased, and productivity is lowered.